Pyrolysis of Early Pre-Cambrian Onverwacht Organic Matter (> 3 × 109 yr old)

2,5-Dimethylfuran from approximately 2.7 x 10-year-old Rupemba-Belingwe stromatolite, Rhodesia: Potential evidence for remnants of carbohydrates

2,5-Dimethylfuran, 2-methylfuran, and C(9)-C(20)n-alkanes have been isolated from the kerogen of a Rupemba stromatolite. These furans could be the oldest known stable remnants of biological carbohydrates if it is possible to prove that their progenitors are as old as the rock. Experiments showed that it is improbable that these compounds are analytical artifacts or laboratory contaminations. Postdepositional contaminations caused by fluids flowing through the rocks seem doubtful because of minimal permeability, which is supported by petrographic observations. Contamination by diffusion also appears to be unlikely. Because the rock had a mild temperature history, these molecules could have survived since Archean time. Because of all the evidence, the furans and alkanes are quite likely biochemical fossils.

Nonprevalence of biochemical fossils in kerogen from pre-Phanerozoic sediments

Evidence of biochemical and geochemical evolution was sought in insoluble carbonaceous matter from 30 selected pre-Phanerozoic sediments ranging in age from about 3.8 to about 0.7 x 10(9) years. The carbon isotope ratios observed were in the range of -20 to -32 per mil with reference to the Peedee belemnite standard, similar to those previously reported. No systematic trends are obvious to us. Stepwise pyrolysis-gas-chromatography showed only molecules with fewer than 8 carbon atoms at the level of sensitivity of 10(-9) g of organics in a 10 mg rock sample. Carbon, hydrogen, and nitrogen analyses showed noncarbonate carbon from less than 0.1% to more than 3%, with very small amounts of N. The H/C (atomic) ratios on HCl-leached and HF-treated samples were generally less than 0.3. Evidence of low pyrolysis yields (micro-analysis) and low H/C atomic ratios (macro-analysis) implies that the carbonaceous solids in even the least metamorphosed of these ancient sediments have evolved far toward amorphous carbon or graphite and do not yield useful "biochemical fossils."

Pyrolysis of Precambrian kerogens: constraints and capabilities

Precambrian kerogens are currently considered to be the primary candidates for the search of biochemical fossils. Degradation of kerogens by relatively "mild" pyrolysis techniques, such as under high vacuum, can liberate indicative structural moieties which were incorporated in, and perhaps shielded by, these solid and highly condensed, basically aromatic substances. It is necessary to observe analytical constraints (sample size and shape, temperature, pressure, time, etc.) in order to prevent an overabundant yield of secondary pyrolyzates (inter- and intramolecular rearrangements) which can prevent kerogen characterization. Potential biochemical fossils have been found in Precambrian kerogens. Demonstratable syngenetic biochemical fossils are expected after kerogen diagenesis and catagenesis is understood in sufficient detail, and when pyrolysis is augmented by multiple, improved analytical techniques.

Carbon Isotopic Studies of Organic Matter in Precambrian Rocks

Reduced carbon in early Precambrian cherts of the Fig Tree and upper and middle Onverwacht groups of South Africa is isotopically similar (the average value of delta(13)C(PDB) is -28.7 per mil) to photosynthetically produced organic matter of younger geological age. Reduced carbon in lower Onverwacht cherts (Theespruit formation) is anomalously heavy (the average value of delta(13)C(PDB) is -16.5 per mil). This discontinuity may reflect a major event in biological evolution.